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426                               CHEMICAL ENGINEERING
the instrument acts as an ordinary galvanometer. The single setting in position 2 reduces the error in the ordinary galvanometer by the factor Ho> which is usually sufficient. The adjustment for the proper external resistance, if desired, can be made with 10 times the precision necessary. Thus if the galvanometer can be read to J-f o of a scale division, the line resistance can be set for an error less than Moo of a scale division, which is at least 10 times the accuracy possible with any indicating instrument. Hence the factor of variable line resistance which may give rise to very serious errors is easily and accurately controlled by a simple mechanical adjustment.
The device is readily applicable to multiple installations of different line resistance. For multiple-point recorders and indicators as many resistances BC may be employed as there are couples. These may be inexpensive rheostats having a resistance of approximately 15 ohms each located in each couple line between the couple and the selective switch. These rheostats may be adjusted in the manner described whenever convenient or necessary. The following illustrates a suitable proportioning of resistance for a 300 ohm indicator.
r2 — 135 ohms
r3 = 150 ohms
n = 15% ohms
rs = 15 ohms 7*2 + r3 + n = 300 ohms
If the simple indicator has the proper ratio of manganin to copper its temperature coefficient is practically zero. In that case the shunt r4 should have the same manganin to copper ratio as the portion of the galvanometer resistance comprised by r3. The complete instrument will have accordingly a zero temperature coefficient. If
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FIG. 6.—Principle of potentiometer.
the simple indicator does not have a zero temperature coefficient it is possible by increasing the proportion of manganin in r4 to compensate for the temperature coefficient of the resistance r3 so that the complete instrument has a zero temperature coefficient.
Potentiometer Method.—The most accurate method for measuring the electromotive force of a thermocouple is by use of a potentiometer. The fundamental principle of this instrument is illustrated by Fig. 6. A constant current from the battery B flows through the slide wire resistance abc. One wire of the couple T is connected to the movable contact b and the other wire, in series with a sensitive galvanometer, is connected to a. The contact b is moved until the galvanometer reads zero thus showing that no current is flowing through the thermocouple circuit. When this balance or zero setting is made, the true electromotive force of the couple is equal to the potential drop across ab. This is obtained from Ohm's law, e = ir, where i is the current flowing through the resistance r = ab.